Adaptive kinetic Monte Carlo simulations of surface segregation in PdAu nanoparticles
Adaptive kinetic Monte Carlo simulations of surface segregation in PdAu nanoparticles
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DOI：
10.1039/c9nr01858a
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发表时间：
2019-06-07
期刊：
影响因子：
6.7
通讯作者：
Henkelman, Graeme
中科院分区：
文献类型：
--
作者：
Li, Lei;Li, Xinyu;Henkelman, Graeme
双金属纳米颗粒的表面偏析对其催化活性至关重要，因为其活性在很大程度上由其表面组成决定。然而，人们对表面偏析的原子尺度机制和动力学知之甚少。原因之一是很难在实验上解决原子重排问题。在原子尺度上模拟表面分离也很困难，因为原子重排可以发生在几秒或几分钟的时间尺度上--比分子动力学可以模拟的时间要长得多。本文用自适应动力学蒙特卡罗(AKMC)方法模拟了PdAu纳米粒子在实验相关时间尺度上的分凝动力学，并在原子水平上揭示了核@壳和随机合金NPs动力学稳定性的根源。我们对PdAu纳米粒子的关注源于实验工作，实验表明，直径小于2 nm的核@壳和无规合金PdAu纳米粒子都是稳定的，这表明其中一种结构一定是亚稳态的，并且是动态捕获的。我们的模拟表明，Au@Pd和PdAu无规合金NPs在400K以下都是亚稳态的，并在数小时的时间尺度上被动力学捕获。这些AKMC模拟提供了对两个NP结构的能量格局的洞察，以及导致偏析的扩散机制。在核壳结构的NP中，表面偏析主要发生在(100)面上，既有空位机制，也有协同机制。当核心处的所有角位都被Pd原子占据时，系统就会被动力学捕获。进一步的偏析需要更高的能垒，因此亚稳态的NP具有部分合金化的壳层。相反，无规合金PdAu Np的表面偏析受到抑制，这是因为与Au@Pd Np相比，无规合金Np的应变减小了，而且合金中的偏析机制需要更多的弹性能来交换Pd和Au，以及表面和亚表面之间的交换。
Surface segregation in bimetallic nanoparticles (NPs) is critically important for their catalytic activity because the activity is largely determined by the surface composition. Little, however, is known about the atomic scale mechanisms and kinetics of surface segregation. One reason is that it is hard to resolve atomic rearrangements experimentally. It is also difficult to model surface segregation at the atomic scale because the atomic rearrangements can take place on time scales of seconds or minutes -much longer than can be modeled with molecular dynamics. Here we use the adaptive kinetic Monte Carlo (AKMC) method to model the segregation dynamics in PdAu NPs over experimentally relevant time scales, and reveal the origin of kinetic stability of the core@ shell and random alloy NPs at the atomic level. Our focus on PdAu NPs is motivated by experimental work showing that both core@ shell and random alloy PdAu NPs with diameters of less than 2 nm are stable, indicating that one of these structures must be metastable and kinetically trapped. Our simulations show that both the Au@ Pd and the PdAu random alloy NPs are metastable and kinetically trapped below 400 K over time scales of hours. These AKMC simulations provide insight into the energy landscape of the two NP structures, and the diffusion mechanisms that lead to segregation. In the core-shell NP, surface segregation occurs primarily on the (100) facet through both a vacancy-mediated and a concerted mechanism. The system becomes kinetically trapped when all corner sites in the core of the NP are occupied by Pd atoms. Higher energy barriers are required for further segregation, so that the metastable NP has a partially alloyed shell. In contrast, surface segregation in the random alloy PdAu NP is suppressed because the random alloy NP has reduced strain as compared to the Au@ Pd NP, and the segregation mechanisms in the alloy require more elastic energy for exchange of Pd and Au and between the surface and subsurface.